1. Field of the Invention
The present invention relates to an optical fiber amp repeater, provided on an optical channel, for amplifying an optical signal in an optical transmission system, and more particularly to an optical fiber amp repeater that transmits signals with an auxiliary signal superimposed on a main signal for the purpose of the maintenance of the optical transmission system.
2. Description of the Related Art
In an optical transmission system, an auxiliary signal is superimposed on a main signal, for the purpose of maintenance, in order to recognize a fault location on the optical channel or to discriminate a fault optical fiber amp repeater. The auxiliary signal is superimposed on the main signal in the optical fiber amp repeater provided on the optical channel. The auxiliary signal includes monitoring information for monitoring the operating state of a terminal unit or an optical fiber amp repeater.
An example of conventional optical fiber amp repeaters of this kind, in which an auxiliary signal is superimposed on a main signal, has shown that the auxiliary signal having wavelength different from the main signal is multiplexed by the wavelength multiplex transmission technique for the data transmission. The conventional optical fiber amp repeater comprises a wavelength multiplex coupler for separating a multiplexed optical signal and composing an optical signal, an optical-to-electrical transducer for converting an optical signal into an electrical signal, and an electrical-to-optical transducer for converting an electrical signal into an optical signal. The repeater takes out the auxiliary signal superimposed on the main signal from input signals, converts it into an electrical signal for processing, further converts it into an optical signal, so as to be combined with the main signal, and sends it to the transmission channel again, thereby transmitting the auxiliary signal.
An example of the conventional technique of this kind is disclosed in Japanese Patent Publication Laid-Open (Kokai) No. Heisei 3-258038, "A Monitoring Method of an Optical Fiber Transmission Channel". This publication describes an optical fiber amp repeater comprising a means for receiving an optical signal modulated by a main signal and reproducing the main signal, a means for obtaining monitoring information out of the reproduced main signal, and a means for generating an optical signal modulated by the different wavelength from the main signal according to the obtained monitoring information, the repeater transmitting the generated optical signal through an optical transmission channel.
Another example of the conventional technique of this kind is disclosed in Japanese Patent Publication Laid-Open (Kokai) No. Heisei 4-306928, "An Optical Repeater and a Monitoring System of an Optical Channel by the Use of the Repeater". This publication discloses an optical fiber amp repeater in which, of the excitation light entered into a rare earth dope optical fiber from an excitation light source through a wavelength multiplex optical coupler, the remaining excitation light after going through the rare earth dope optical fiber is separated by means of the wavelength multiplex optical coupler so to enter one into a light excitation laser, thereby to generate a monitoring signal light having the wavelength different from that of the main signal light within the gain-wavelength of a rare earth dope optical fiber and combine the monitoring signal light with the main signal light having been amplified by an optical coupler to provide.
Another example of the conventional optical fiber amp repeater has shown the optical repeater that superimposes an auxiliary signal on the envelope of the peak value of the main signal by modulating the excitation light source by the auxiliary signal. Japanese Patent Publication Laid-Open (Kokai) No. Heisei 4-3028, "An Optical Repeater" discloses the conventional technique of this kind. It describes an optical fiber amp repeater comprising an oscillator for modulating bias current of a pump light source in an optical fiber amplifier by low frequency wave, an optical wave combining device connected to the output of the optical fiber amplifier, an optical detector for detecting a part of output optical signal of the optical wave combining device to convert into an electrical signal, a band-pass filter permitting the signal having the frequency similar to the output frequency of the oscillator, of the output signals from the optical detector, and an automatically gain control circuit for controlling the gain of the optical fiber amplifier so that it may be constant by adjusting the bias current of the pump light source according to the output signal of the band-pass filter.
The above-mentioned optical fiber amp repeater, however, has the following problems. The first example of the conventional technique, in which the optical repeater is operated with an auxiliary signal converted into an electrical signal, may be realized by an optical fiber amp repeater of a simple structure. However, it requires an optical-to-electrical conversion circuit in order to convert an auxiliary signal into an electrical signal for the processing, as well as an electrical-to-optical conversion circuit in order to further convert the processed electrical signal into an optical signal for the transmission, thereby increasing the manufacturing cost.
The second example of the conventional technique, in which the excitation light source is modulated by the auxiliary signal, has to modulate the excitation light source by low frequency wave according to the characteristic of the optical fiber amp repeater, thereby resulting in restricting the capacity of transmitting the auxiliary signal.
Further, the above second example of the conventional technique varies the output level of the main signal to a degree because an auxiliary signal is superimposed on the envelope of the peak value of the main signal, thereby resulting in spoiling the original function of amplifying the main signal in some degree.